Capacity switch



May 21', 1946- o. M. wooDwARD CAPACITY SWITCH Filed Feb. 27, 1945 2 Sheets-Sheet 1 Inventor Gttorneg Patented May 21 1946 CAPACITY SWITCH Oakley M. Woodward, Princeton, N. 1.. assignor to Radio Corporation of America,a corporation of Delaware Application February 27, 1943, Serial No. 427,482

(or. nit-44) 3 Claims.

This invention relates to switches for controlling radio frequency energy and more particularly to capacitor switches of the type which are combined with resonant transmission line sections to provide an open circuit or short circuit at a predetermined point in a qency distribution system.

It is the principal object of the present invention to provide an improved method of and means for producing a short circuit or open cir cuit at a point on a transmission line.

radio freimum to minimum capacitance is low, the adjustments of the lengths of the line sections 3 and 9 are very critical. Thi ratio is governed by the physical dimensions and the spacings of the capacitor plates, and there is a fairly low practical limit to which the ratio can be increased by employing large plates or close spacing.

A further object is to provide a capacitor having a very high ratio of maximum reactance to minimum reactance.

Another object is to providean improved capacitor type radio frequency switch.

The foregoing and other objects will become apparent to those-skilled in the art upon consideration of the following description with reference to the accompanying drawings, of which Fig. 1 is a schematic diagram of a capacitor According to the present invention, a parallel resonant circuit is connected in series with the capacitor. The tuning of this circuit is adjusted to provide inductive reactance at the fre quency of the energy to be controlled. The total effective reactance of the parallel resonant circuit and the capacitor may be adjusted to any desired value by tuning the parallel circuit.

Thus the maximum effective capacitance may be made very larg without requiring large plate switch constructed according to prior art practice, Fig. 2 is a sectional view of a variable capacitor constructed according to the-instant invention, Fig. 3 is an elevation taken along the section 3-3 of Fig. '2, and Fig. 4 is a schematic diagram of the equivalent circuit of the device of Fig. 2.

Referring to Fig. l, a known type ofradio frequency switch comprises a variable capacitor 1 connected through a. transmission line section 3 to a point 5 on a transmission line 1. A short circuited stub line 9 is also connected to the point 5. The length of the line section 3 is such that the maximum capacitance of the device I is transformed to a short circuit at the point 5. The stub 9 is designed to provide the proper amount of inductive reactance at the point 5 to resonate with the transformed minimum ca- Dacitance of the device I. This parallel resonant combination acts substantially like an open circuit across the line 1 at the point 5. Thus as the capacitor 1 is varied between its minimum and maximum values, the point 5 is alternately open circuited and short circuited. Itis to be noted that this type of operation will occur only at the frequency which the line sections 3 and 9 are correctly proportioned with respect to the characteristics of the capacitor I.

It is found in practice that if the ratio of maxareas or close spacing. At the same time the minimum capacitance is substantially unchanged.

Referring to Fig. 2, a preferred embodiment of the invention comprises a cylindrical housing ll, provided with circular end plates I3 and I5, and an internal supporting plate ii. The plates I3 and is are provided with central apertures for bearings l9 and 2! respectively. A plug 28 is supported in the bearing l9 and carries a shaft 25. The plug '23 also supports a tubular member 21 at one end. The other end of the member 21 issupported by the bearing 2|. A clamping ring 29 is supported at the end of the tubular member 21 near the bearing 2!. The shaft 25 extends through the tube 21 and terminates in a reduced portion 3| which is threaded. A clamping disc 33 is provided upon the reduced portion 3| for engagement with a disc 35 of insulating material. A metal disc 31 is supported between the insulating disc 35 and the clamping ring 29. One-half of the disc 35 is coated with alayer 39 of conductive material, as more clearly shown in Fig. 2. The disc assembly is held together rigidly by means of nuts Al on the threaded portion 3!! of the shaft 25. A distorted washer 42 of spring material is provided to insure connection between the clamping ring 29 and the plate II. The tubular member 21 is provided with a longitudinal slot 43. A shortingplug 45 is supported on the shaft 25 andis provided with set screws 41 and 49, adapted to v engage the tube 21 and the shaft 25 respectively.

A cage-like structure 5| of spring material is secured to the plug 45 to provide contact to the shaft'and the inner wall of the tube 2|.' A pair of stationary plates 53 are supported on insulating bushings 55 which extend through and are supported by the end plate I 5. Connections to the plates 53 are made through conductors 51' extending through the bushings 55.

The tubular member 21 and the shaft 25 func- I tion as a concentric line, short circuited by the plug 45. The reactance of a short cirouited line is:

tan

where Z is the characteristic impedance of the line, I is the length of the line, and A is the wavelength. This reactance is shunted by the capaci the semicircular rotor plate 39 and one of the.

This provides a very low impedfixed plates 53. ance path for the flow of radio frequency current between the fixed plate 53 and the supporting plate H. Inasmuch as the plate I! is grounded through the housing I I, the plate 53 is effectively grounded for currents of the frequency for which the plug 45 is adjusted. As the plate 39 is rotated, the reactance between the plate 53 and the plate I! varies between a relatively high value corresponding to the stray capacitance between the plate 53 and the remainder of the device and a very low value equal to the series resonant impedance of the variable capacitance and the concentric line.

Thus the invention has been described as an improved radio frequency switch of the variable capacitor type. In order to provide a high ratio of maximum reactance to minimum reactance, an inductance is connected in series with the variable capacitor. This inductance is adjusted to the proper value to provide series resonance with the maximum capacitance at the frequency of the energy with which the switch is to be used. An-

important advantage of this arrangement arises from the factthat relatively small capacitor plates may be employed. This decreases the length of the periods when the reactance is neither maximum nor minimum, but at some intermediate value. It should be noted also that the stub 9 of Fig. 1 may be omitted, as shown in Fig. 4, by choosing the lengths of the lines 3 to provide open circuits across the junctions 5 with the minimum capacitances between plates 39 and 53, and adjusting the inductance of the line to provide short circuits at the points 5 with the maximum capacitances between plates 39 and 53. The described embodiment of the invention comprises a rotary capacitor with a short circuited concentric line employed as the inductor means. This arrangement provides a. relatively simple structure having good mechanical strength and a minimum of rotating contacts.

The invention covered herein may be manufactured and used by or forthe Government of the United States for governmental, military, naval and national defense purposes without payment to me or assigns of any royalty thereon,

I claim as my invention:

. 1. A radio frequency switch comprising a substantially cylindricalhousing provided with conductive end plates and an internal suporting plate, a

a pair of conductive sectors supported on insulating bushings extending through one of said end plates, a rotatable 'shaftextending through the other of said end plates and through said internal supporting plate, a tubular member of conductive material surrounding said shaft and electrically connected to said housing, a semi-circular plate of conductive material secured at one end of said.

shaft in proximity to said conductive sectors, and a plug of conductive material adjustably supported on said shaft within and in contact with said tubular member.

2. The invention as set forth in claim 1 wherein said plug is adjusted to such a position longitudinally of said shaft as to provide an inductive reactance substantially equal in magnitude, at a predetermined frequency, to the minimum capacitive reactance between said semi-circular. plate and one of said sectors.

3. The invention as set forth in claim 1 wherein a tubular body of insulating material is included in the space between said shaft and said conductive tube and between said plug and said semicircular plate.

OAKLEY M. WOODWARD. 

